Electro-optical system



July 15, 1941- F. GRAY ELECTRO-OPTICAL SYSTEM Original Filed March 4, 1936 2 Sheets-Sheet l QM vm mm. Nm.

/NVENTOR By E GRA l" A TTORNE Y ImN F.. GRAY ELECTRO-OPTICAL SYSTEM July l5, 194i.

' original Filed March 4, 193e zvsheets-sheet 2 /N VENTOR By EGR/1y MWh/Q Patented July 15, 1941 UNITED STATES PATENT OFFICE ELECTRQ-OPT'ICAL SYSTEM Frank Gray, New York, N. Y., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York 8 Claims.

This invention relates to electro-optical systems and more particularly to means for setting up electric currents representative of the various light-tone values Aof an object, as in television scannine.

Certain light sensitive elect-ric materials will have a large response to light oi Wave-lengths Within a vcertain range and Zero tor negligibly small response in another range or ranges. It has been discovered that this phenomenon can be put to use in electro-optical systems, as for example, television transmission systems, so that a light sensitive electric image receiving plate or layer may be associated with a second light sensitive electric layer adapted to be scanned by a beam of light to produce image currents dependent upon the light intensities of the elemental areas of the image plate, the material of one or both layers being chosen with regard to the wave-length range of Sensitivity, and the wave-lengths of either the scanning beamer the image light beam, or both, also being chosen so that one or the other of these beams may pass through one layer to the other without affecting it, or each beam may impinge upon bot-h layers H while aiecting only one, or each beam may impinge upon a diilerent layer, thus each beam affecting only one.

An object of this invention is to provide apparatus for utilizing the phenomenon mentioned above in the production of electric currents under control of light.

Another object is to provide novel means for producing image currents for use in systems of television and telephotography.

A further object of this invention is to pro vide scanning apparatus which, while not utilizing the phenomenon mentioned above, provides means for making cach film or layer responsive t0 a diiTerent beam of light,

In accordance with one embodiment of the invention, cho-sen for purposes of illustration and hereinafter described in detail, a gas-tight container enclcscs a target, comprising a metallic plate carrying a photo-conducting lm, i. e., a

film formed of material adapted to have its conductivity increased when supplied with radiations of one Wave-length range, on which is applied a very thin lm or coe-ting of a non-conducting phcto-emissive material adapted to emit electrons when exposed to radiations of a second wave-length range which radiations, however, do not affect the conductivity of the photo-conducting film., and a fine mesh electrode adjacent the coating oi photo-emissive material. Associated with the container is a means for applying radiations of the rst wave-length range to an object and for directing radiations reflected from the object through the nterstices of the mesh electrode and the photo-emissive layer, upon the photo-conducting lm to control its conductivity. Also associated with the container is a means for producing a moving beam of radiations of the second Wave-length range and for causing it to pass through the interstices of the mesh electrode to scan successively the elemental areas of the photo-emissive coating, Which is thereby activated to cause the production of a moving beam of electrons which passes to the mesh electrode to thereby cause a flow of image current to `an external circuit. The flow of photoelectrons from an elemental area ei the photoemissive coating is controlled by the conductivity imparted to successive elemental areas of the underlying photo-conducting layer by radiations corresponding to the lights and shades of the object.

As an alternative, radiations reflected from the object may be applied to the photo-emissive coating kin radiations which Vcause it to emit electrons but which do not affect the conductivity of the photo-conducting film and an imag-e current may be produced by scanning the photo-conducting film with a beam vof radiations which renders its elemental areas conductive in turn but which does not cause the photosemissive coating to emit electrons.

In another embodiment, the target may coinprise a metal plate, a pair of photo-conducting films of materials respectively sensitive to different Wave-length ranges of radiations, and a transparent conductive layer. Radiations of one range reiected from the object control the conductivity of the elemental areas of one of the films in accordance with the iight-tone values oi the corresponding elemental areas of the object While a beam of radiations of a second Wavelength range scans in succession the elemental areas of the second nlm through the transparent layer and the 4first film to successively render these elemental areas conducting. The external circuit is connected to the metal plate and the transparent conductive layer, so that when the elemental areas of both films are made conducting by the radiations projected on them respectively, an image current which varies with the tone values of the object passes through this eX- ternal circuit.

As alternative to this embodiment, the tar vget may lcorriprise a transparent plate provided with a transparent metal coating carrying two superposed lms of photo-conducting material and a transparent conducting layer, the image controlled radiations being applied to one lm from a position in front of the target to control the conductivity applied to its respective unit areas, as described above, while the scanning beam of radiations of a different wave-length range is directed through the transparent plate to the other lm from a position behind the target. The external circuit is connected between the metal coating and the conducting layer and an image current is supplied to that circuit in a manner similar to that described above. rlhe target may be applied to the external surface of the end wall of a cathode ray tube, i. e., the wall carrying the fiuorescent screen, and radiations froin the fluorescent spot may be used to effect scanning. Again, the target may comprise two photo-conducting lms of the same material separated by an intervening black, high resistance layer, which prevents the scanning spot from affecting the photo-conducting film to which the image controlled radiations are applied. As a further alternative, a single lm of photo-conducting material may be used, provided it is just thick enough to prevent the scanning spot and the radiations controlled by the object from affecting it throughout. In either of the two last-mentioned alternatives, radiations of the same type may be used both for illuminating the object and for scanning the target.

If the photo-conducting layers in any of the above embodiments are made of a material having high specific resistance and are made relatively thin compared to the size of the area covered by the scanning light beam, they may be made continuous as the lateral conductivity therethrough will be so small as to be negligible. rl'he thin film and the high specific resistance help to produce a large stored charge which is discharged by the scanning beam.

The invention will be more readily understood by referring to the following description, taken in connection with the accompanying drawings forming a part thereof, in which:

Fig. l is a diagrammatic representation of a television system including the invention;

Fig. 2 is an enlarged perspective view of the .g

assembly used in the scanning tube of Fig. 1;

Fig. 3 is an enlarged perspective View of a type of assembly that may be used as an alternative to that shown in Fig. 2;

Fig. 4 shows a system somewhat similar to that of Fig. l embodying a modified method of scanning;

Fig. 5 is an enlarged perspective View of the assembly used in the scanning tube of Fig. 4;

Fig. 6 shows an image target mounted on the end of a cathode ray tube; and

Figs. 7 and 8 are modifications of Fig. 5.

Referring more particularly to the drawings, Fig. l shows a television system which, in general, comprises a television transmitter T connected by a transmission channel L to a television receiver R.

The transmitter T includes a scanning tube S, an optical system for gathering radiations of one type, reflected from an object O, and for directing .ff

these radiations to a target included in the scanning tube, and means, such as the cathode ray tube C, for producing a moving beam of radiations of a second type and, in cooperation with a suitable optical system, for causing this beam to scan the target. Scanning tube S comprises a gas-tight container I0 of glass or other suitable material enclosing an image target or screen, represented generally as II, and a ne mesh electrode I2 spaced therefrom.

In order to clarify the detailed description of the system of Fig. 1, reference will now be made to Fig. 2, which illustrates an enlarged perspective View of the assembly used in the scanning tube S, with the glass container broken away. As therein shown, the assembly comprises a suitable anode, such as the mesh electrode I2, and a target or screen II comprising an order (starting from the side away from the electrode I2): a metal plate I3 carrying a photo-conducting film I4 which in turn carries a coating I5 of photoemissive material. The conductivity of the photo-conducting nlm I4 increases when supplied with radiations of one wave-length range, which for convenience will be hereinafter designated A-radiations, while the photo-emissive coating I5 is sensitive to a second wave-length range of radiations, hereinafter designated B-radiations, which do not affect the conductivity of the lm I4.

The iine mesh electrode I2 is adjacent the photo-emissive coating I 5 and is connected to the meta-l plate I3 by an external circuit, including a battery I6 and a resistance I1, the latter being included in the input circuit of an amplifier I8. The negative terminal of the battery IS is connected to the metal plate I3 and the batterys positive terminal is connected to the mesh electrode I2 through the resistance I 7, whereby this electrode is polarized to operate as an anode with respect to the plate I3.

The object or iield of view O is supplied with radiations from a suitable source, not shown, and these radiations are reflected from the object through an optical system, represented generally by a lens I9, through the interstices of the mesh electrode I2 and through the thin coating I 5 onto the photo-conducting film I4 where an image of the object O in A-radiations is formed. The

' small areas of the lm I4 are thereby successively rendered conducting in accordance with the tone values of the corresponding small areas of the object field O. A suitable optical iilter 20 may be inserted between the object O and the lens I 5 in order to pass only the type of radiations desired.

Also associated with the scanning device Sis a second optical system represented by a means for generating a moving spot of light and a lens system, represented generally by a lens 2 I, for forming a moving beam of radiations to scan the surface of the photo-emissive coating I 5. The means for generating the moving spot of light is represented and will be described below as a cathode ray tube C having a fluorescent screen 22, but any other suitable type of apparatus for producing this result may be used, as for example, a source of radiations associated with a disc having a series of apertures arranged in a spiral.

The cathode beam of the tube C is deflected in two directions at right angles to each other and at such relative speeds in the two directions that the screen 22 is excited to iiuorescence to produce a moving spot of radiations which covers the entire screen in a time interval within the period of persistence of vision, in order to prevent flicker. Deiiection of the beam in one direction is effected by the :field between a pair of defiecting plates 23 supplied with current having a sawtoothed wave form, the production of this current .being controlled by oscillations of line scanning frequency supplied by the oscillator 24 to the deflecting-current producing device 25. Deflection in the other direction is produced by supplying a second pair of deflecting plates 26 with a current, also of saw-tooth wave form, supplied by the deilecting-current producing device 2l, controlled by oscillations oi image cycle frequency produced by a subharmonic generator 23 supplied with oscillations from the oscillator 24. Any suitable apparatus, such as that disclosed in United States Patent 1,613,954, January 11, 1927, to Knoop, may be used to produce the deflecting currents.

The operation of the scanning tube S is as follows:

A-radiations from a suitable source (not shown) are reflected by the object O and projected through the interstices of the mesh electrode I2 and the photo-emissive coating I5 uph the photo-conducting film i4. These A-radiations increase the conductivity of the lm I4 but do not affect the layer I5. A beam of radiations frcm the moving spot of light produced by the fluorescent screen 22 is directed by the lens system 2l through the interstices of the mesh electrode I2 to scan successively the elemental areas of the photo-emissive coating I5. These radiations are of the second, or B-type, and cause the coating I to be activated to cause the flow of a moving beam of photoelectrons to the mesh electrode l2, the flow from any elemental area being controlled by the conductivity imparted to the corresponding elemental area of the photo-conducting film I 4 by the radiations reflected by the object O. In other words, the resistance of the circuit through the tube at any instant is dependent upon the resistance of the elemental area of the photo-conducting film I4 which corresponds to the elemental area of the photo-emissive coating I5 being scanned at that time. The resistance of the elemental areas of the film I4 is determined by the intensity of the radiations reflected from the object O` onto the successive elemental areas of the film. Thus, an image current varying with the tone values of successive elemental areas of the image O is caused to fiow through the external circuit including a resistance II, as described above, where it is supplied, after amplication by the device I8, to a transmission circuit and over a line L, or a line carrier or radio channel, to a distant receiving station which includes a receiver R.

Receiver R may include an amplifier, in case the image current is directly transmitted, or a demodulator and ampliiier in case transmission is effected in accordance with line carrier or radio practice. It may also include a cathode ray discharge device 33 comprising a cathode 3| and an anode 32 for producing the cathode beam, two pairs of plates 33 and 34 for respectively effecting deflection of the beam in two directions at right angles to each other, a fluorescent screen 35, and a pair oi control grids 35 and 31 connected by an external circuit 38 to which the image currents received over line L and amplied by the device 33 are supplied by means of a transformer 43. Auxiliary devices 4I, 42 and 43, similar to those described in connection with the transmitter, are also used in the receiving system R. Devices 4l and 42 operate, as disclosed in the Knoop patent, to supply deiiecting current of saw-tooth wave form to respective pairs of deflecting plates 34 and 33. The apparatus for producing the deflecting currents is controlled by current of line scanning frequency received from the oscillator 24 via a transmission line or radio channel, which has not been shown, but its feeders have been designated as L' and L. In this manner, the cathode beam of the receiver R is deflected in synchronism and in phase with the deiieotion of the cathode ray beam of the discharge device C used at the transmitter.

The cathode ray discharge device 36 at the receiving station operates in the following manner:

The control electrodes or grids 36 and 31 are closely adjacent each other in a position between the anode 32 and the fluorescent screen 35 and comprise segments of a sphere, the centers of which are close to the centers of the deflecting fields produced by the pairs of plates 33 and 34. The grid 36 is maintained at substantially the same potential as the anode 32 and the signal potentials are applied to the grid 31, which may be negatively polarized with respect to the grid 36. The two grids, therefore, serve to define a very limited zone in which the signal potentials are effective for controlling the cathode beam, and this zone is substantially isolated from the equipotential section established between the anode 32 and the grid 36, within which zone deflection of the beam is effective. Consequently, deflection of the beam is controlled by the fields established between the pairs of plates 33 and 34 without being influenced by the signal potentials, and the signal potentials operate to control the velocity or number of electrons constituting the beam and hence the intensity of the excitation of the uorescent screen, which determines the quality of the image produced, without causing the direction of travel of the electrons to be varied. In other words, the two sets of elements for eiectively controlling the deflection and intensity of the beam, each of which tends to interfere with the other and thereby cause loss of focus of the beam and distortion of the image, are so positioned and electrically controlled as to materially, if not completely, avoid such interference. For a more complete disclosure of the construction and method of operation of the image producing cathode ray device 30 brieily described above, reference may be made to Patent No. 2,155,192, issued April 18, 1939, to J. B. Johnson.

While a receiver of the cathode ray discharge type has been described, it will, of course, be obvious that a glow lamp cooperating with a revolving disc having a spiral of apertures, or any other well-known receiver may be used with my invention. A satisfactory receiver of the glow lamp type is disclosed in United States 1Batent 1,728,122, September l0, 1929, to Horton.

The A-radiations may be red or infra-red, or in other words, those radiations having a wavelength greater than about 6500 Angscrom units, while the B-radiations blue, violet, or ultra-violet, or in other Words, those radiations having a wave-length less than about 4760 Angstrom units. The wave-lengths given above are merely by way of example and this invention is not speciiically limited thereto. While in Fig, l a system has been described in which the object has applied thereto A-radiations while` the seanning beam is of B-radiations, it is to be understood that B-radiations may be used to project an image of the object onto the photo-eniissivcI coating I5, whereby it is actuated to cause the emission of electrons, these B-radiations not affecting the conductivity of the photo-conducting lm I4; and the ilm I 4 may be scanned with a beam of A-radiations which affects its conductivity but which does not cause electronic emission from the layer I5. The scanning beam in this arrangement renders the elemental areas of the lm I4 successively conductive as they are scanned in turn.

The cathode ray tube is well suited for use as a source for supplying a scanning beam of B- radiations as the spot of light given off by the fluorescent screen is rich in radiations in the blue, violet and ultra-violet portions of the spectrum. If ultra-violet radiations are used for the scanning beam, the lens in the optical system 2I as well as the material of the enclosure should be of a suitable substance to pass the radiations such as, for example, quartz. Also, it might be desirable in some cases to include within the ptical system represented by the reference character 2| a suitable optical lter to filter out all undesired radiations.

The assembly shown in Fig. 2 may comprise a lm I4 of selenium or other suitable material made oi discrete particles so that the lateral conductivity is negligibly small. The film may be in globular form. Selenium is particularly sensitive to red and infra-red radiations. The very thin, and hence, discontinuous transparent, photo-emissive coating I5 may be made of a suitable form of an alkali metal sensitive to blue, violet, or ultra-violet radiations, such as for example, potassium hydride. The iilm I4 may comprise a thin, continuous iilm which may have its lateral conductivity decreased by cross-hatching the i'llm with fine lines through to the metal plate I3, or the surface of the metal plate may be cross-hatched with numerous, deep, and narrow grooves.

The operation described above may be called the resistance method. If the thin film I4 is of sufficiently high specific resistance, the device will operate by another method, that is, by the so-called storage method because charges are laid down on one surface of the photo-conducting element which leak oir during the next image cycle, the recharging current thus forming a greatly increased image current. This high specic resistance also makes possible the use of a continuous lm of photo-conducting material while still restraining the lateral conductivity to t a negligible minimum, provided that the thickness of the lm is small compared to the diameter of the scanning spot, that is, for example, one-tenth of that diameter. For a more complete description of the storage method of operation .pf

in comparison with the resistance method, and the relation of lateral to transverse conductivity in thin high resistance films reference may be rnade to Patent 2,195,486, issued April 2, 1940, to Frank Gray. A suitable photo-conducting material which is of sufliciently high specific resistance to operate in occordance with the storage method and which is sensitive to red or infra-red radiations is mercuric iodide (HgIz).

An enlarger perspective View of a modied scanning tube is shown in Fig. 3. In this embodiment there is no mesh electrode, the external circuit being connected between the metal plate and a light transmitting conducting layer on the target. The composite target 4l comprises a metal plate I3 carrying two photo-conducting i'ilms 44 and 45 of different materials and a transparent conducting layer 46. The conductivity of the photo-conducting film 44 is changed by B-radiations, but is not affected by A-radiations. In the case of the photo-conducting :film 45, the reverse is true as its conductivity is changed by A-radiations, but is unaiected by B-radiations. A-radiations reilected by the object are projected through the conducting layer 46 onto the rst lm 45 and a beam of B-radiations passes through the conducting layer and the first lm to scan the second lm 44, whereby a moving conductive path is provided between the conducting layer 46 and the metal plate I3, the external circuit is completed, and an image current, controlled by the variations of conductivity successively imparted to small areas of the rst iilm by A-radiations, flows through the external circuit. If the second film 44 is made very thin and of high specific resistance when dark (that is, when not illuminated with the radiations to which it is sensitive), there is provided between the first iilm 45 and the metal plate I3 a capacity in which image impulses, corresponding to the elemental areas of the image, may be stored and hence a greatly increased image current ilow is produced in the external circuit, which circuit is similar to that shown in Fig. l. If the A-radations are red or infra-red, the photo-conducting film 45 may be selenium for the resistance method or mercury iodide for the storage method and a photo-conducting film 44, sensitive to B-radiations (blue, violet, or ultra-violet), may consist of thallium bromide or chloride which may be used for either method as the specic resistance of the lm 44 when illuminated does not have to be as high as that of the iilm 45 as this film 44 is used to discharge the stored charge rather than to store it up.

Fig, 4 shows a scanning tube 8', which is adapted t-o be used in cases where the image is projected from ra position in front of the target and a scanning beam is applied from a position behind the target. Referring more particularly to Fig. 5, which is an enlarged perspective view of the assembly within the tube Si, this tube comprises an evacuated container in which is mounted a composite target 59, comprising a transparent supporting plate 5I of glass or similar material coated with a transparent metal layer 52 of a suitable substance such as silver, films 44 and 45, and a transparent conducting layer 4S. The lms and the conducting layer are similar to those described above with reference to Fig. 3. The external circuit is connected between the transparent metal layer 52 and the conducting layer 46. The scanning beam is produced by the iluorescent screen of the cathode ray tube C, this and the optical systems I9 and 2I being also similar to those disclosed in connection with Fig. l. Image controlled radiations are projected through the transparent conducting layer 46 onto the photo-conducting lm 45 to control its conductivlty n accordance with the tone values of the object. The scanning beam B from the iluorescent screen of the cathode ray tube C passes through the transparent plate 5I and the transparent conducting layer 52 to scan successively the elemental areas of the photo-conducting lm 44 to impart conductivity to these elemental areas in succession. An image current is, therefore, produced which ilows through the extern-al circuit in a manner similar to that described above. This arrangement will also operate in accordance with the storage method described above.

Figs. 6, 'l and 8 each includes a target, similar to that disclosed with reference to Fig. 5 except for the differences noted below in the description of each figure, mounted on the external surface of the end wall `6i! of the scanning cathode ray tube C.

In Fig. 6, the end wall 'Eil takes the place of the glass plate l of Fig. 5, but the target is otherwise similar. It comprises a transparent metal layer 52, preferably of silver, two photo-conducting films 44 and 45 of different materials, respectively similar to the photo-conducting lms described in connection with Fig. 3, and a conducting layer 46 of silver or the like. While the glass enclosure 6l for the target is shown attached to the end wall of the cathode ray tube C, it may be spaced therefrom and have the target mounted in a position intermediate lthe walls of the container 6l. This construction would include a glassr plate similar to the member 5| of Fig. 5. The operation of the device shown in this figure is similar to that described above with reference to Fig. 4.

In Fig. 7, the two photo-conducting films are made of the same material and are separated by a medium to prevent radiations applied to one film from affecting the other film. The target thus comprises a transparent metallic layer 52,-

photo-conducting iilms 54 and 55, a thin, black, high resistance layer 55, made yof Bakelite or similar material, placed between the two photo-conducting films to prevent radiations applied to the lm 55 vfrom affecting film 54, and vice versa, and a conducting layer 45. With this construction, the same type of radiations may be used to both illuminate the image iield and to scan the target. The method (or methods) of operation is similar to that of Fig. 6. The resistance layer 55 is so' thin and of such high specific resistance that its lateral conductivity is small with respect to its transverse conductivity.

Fig. 8 shows a target in which a single film 62 is mounted between the conducting layers 52 and 45 and it is just thick enough to prevent both the scanning beam and the radiations applied to the image field from affecting it throughout. This design permits the use of the same type of radiations for both illuminating the field and scanning the target. The film 52, as Well as the iilms 54 4and 55 of Fig. 7, may be made of the same material as either nlm 44 or 45 of Fig. 3, depending on the type of radiations it is desired to use. The operation ofv the device shown in this figure is similar to that described above in connection with Fig. 7, the single thick iilrn 62 of this figure performing the functions of both iil'ms 54 and 55 of Fig. 7. In this iigure, as well as in Fig. 7, the glass enclosure 6| has been omitted but it is preferable to have such an enclosure to protect the thin layers or films compr-ising the target.

In Figs. 6 to 8, inclusive, an optical system I9, similar to that shown in Fig. 1, may be used for projecting radiations from the image O on the target. It would also be satisfactory to use the same typ-e oi external circuit as used in .that figure.

While this invention has been shown as applied to the situation where the object is a human being, it will be apparent that the invention is also applicable to the transmission of outdoor scenes or motion picture films, either using radiations reiiected from the film, or passing through it, to control the operation of the scanning screen or target. n

While several specific embodiments for utilizing the invention have been disclosed, it is to be understood that this invention is not limited thereto, but only by the scope of the appended claims. In these claims, the term sensitive to radiations as used to describe certain elements is intended to include both photo-conducting and photo-emissive materials and the terms illuminating and light are intended to apply only to electromagnetic radiations as distinguished from a beam of electrons. The term path which conducts electrons is intended to include the case in which the two photo-conducting layers are in actual contact or in which they are connected by electrical conductors, and also the case in which the two layers are separated by a medium or" high specific resistance but of such thinness that its transverse conductivity is suiiiciently large that it conducts electrons between the photoconducting layers.

This application is a division of application Serial No. 226,633, filed August 25, 1938i, which in turn is a division of Patent 2,150,159, issued March 14, 1939i, on an application filed March 4, 1936 (Serial No. 67,059).

What is claimed is:

l. An electro-optical arrangement comprising a target including a first photoconducting layer and a second photoconducting layer near said rst layer and cooperating therewith, a single circuit connection to all of the elemental areas of said iirst photoconducting layer, a second single circuit connection to all of the elemental areas or said second photoconducting layer, each of the various elemental areas of the first photoconducting layer being connected to the corresponding elemental areas of the second photoconducting layer by a path which conducts electrons, means for illuminating said first layer with radiations i'rom an object, said second layer not being directly aiected thereby, and means on the side of said target remote from said illuminating means for scanning said second layer with a beam ci light radiations, said first layer not being directly afiected thereby, whereby the radiations from the object act on the rst layer to condition the arrangement and the scanning beam of radiations acts on the second layer, the two groups of radiations cooperating in the formation of an image current.

2. In combination, a first plate-like photoconducting member, the thickness of which is only a ver'y small fraction of either of the two dimensions of its front and rear laces, a second Platelike photoconducting member of approximately the same dimensions as said first photoconducting member positioned substantially parallel to said first member, a single circuit connection to all of the elemental areas of said first photoconducting member, a second single circuit connection to all of the elemental areas of said second photoconducting member, each of the various elemental areas of the rst photoconducting member being connected to the corresponding elemental areas of the second photoconducting member by a path which conducts electrons, means for illuminating said first member with radiations from an object, means on the side of the second member remote from said first member for scanning said second member with a beam of light radiations, and means for preventing radiations from the object from reaching the second member and for preventing scanning radiations from reaching the first member, whereby the radiations from the object act on the first member to condition it and the scanning beam of radiations acts on the second member, the two groups of radiations cooperating in the formation of an image current.

3. In combination, a first plate-like photoconducting member, the thickness of which is only a very small fraction oi either of the two dimensions of its front and rear faces, a second platelike phctoconducting member of approximately the same dimensions as said rst photoconducting member positioned substantially parallel to said rirst member, a single circuit connection to all of the elemental areas of the first photoconducting member, a second single circuit connection to all of the elemental areas of the second photoconducting member, means for illuminating said first member with radiations from an object, means on the side of said second layer remote from said first member for scanning said second member with a beam of light radiations, and opaque means between said photoconducting members to prevent radiations from the object from reaching the second member and to prevent scanning radiations from reaching the first member, said opaque means serving elemental areas of said second photoconducting layer, each of the various elemental areas of the rst photoconducting layer being connected to the corresponding elemental areas of the second photoconducting layer by a path which conducts electrons.

fl. In combination, a rst plate-like photoconducting member, the thickness of which is only a very small fraction of either of the two dimensions of its front and rear faces, a second platelike photoconducting member of approximately the same dimensions as said iirst photoconducting member positioned near said rst member and substantially parallel thereto, said iirst memer being sensitive to a range of radiations to which said second member is substantially insensitive and said second member being sensi-- tive to a range of radiations to which said rst member is substantially insensitive, means for illuminating said rst layer with radiations from an object from said range of radiations to which said first member is sensitive, and means on the side oi' said second member remote from said iirst member for scanning said second member with a beam of light radiations from said range to which said second member is sensitive.

5. An electro-optical arrangement comprising a target including a rst photoconducting layer and a second photoconducting layer near said rst layer and cooperating therewith, means for illuminating said first layer with radiations from an object, said second layer not being directly affected thereby, and means on the side of said target remote from said illuminating means for scanning said second layer with a beam of light radiations, said first layer not being directly affected thereby, whereby the radiations from the object act on the nrst layer to condition the arrangement and the scanning beam of radiations acts on the second layer, the two groups of radiations cooperating in the formation of an image current, at least one of said photoconducting layers having its lateral conductivity decreased by cross-hatching with ne lines.

6. An electro-optical apparatus comprising a container enclosing a first photoconducting layer, a second photoconducting layer substantially parallel to said rst photoconducting layer and being separated by a distance no greater than the order oi' thickness of either of said layers, means i'or illuminating said first layer with radiations from an object, said second layer not being drectiy aiiected thereby, and means on the side of said photoconducting layer remote from said illuminating means for scanning said second layer with a beam of light radiations, said ii'rst layer not being directly aected thereby, whereby the radiations from the object act on the first layer to condition the apparatus and the scanning beam or radiations acts on the second layer, the two groups of radiations cooperating in the formation of an image current.

'1. An electro-optical arrangement comprising a target including a first photoconducting layer and a second photoconducting layer near said rlrst layer and cooperating therewith, each of the various elemental areas of the first photoconducting layer being connected to the corresponding elemental areas of the second photoconducting layer by a path which conducts electrons, means for illuminating said first layer with radiations from an object, said second layer not being directly arlected thereby, means on the side of said target remote from said illuminating means for scanning said second layer with a beam of light radiations, said rst layer not being directly ailected thereby, whereby the radiations from the object act on the rst layer to condition the arrangement and the scanning beam of radiations acts on the second layer, the two groups of radiations cooperating in the formation of an image current, a light transmitting metallic element in contact with the surface of each of said photoconducting layers remote from the other photoconducting layer, and circuit connections between said metallic elements through which connections the image current passes.

8. An electro-optical arrangement comprising a target including a first photoconducting layer and a second photoconducting layer near said rst layer and cooperating therewith, each of the various elemental areas of the rst photoconducting layer being connected to the corresponding elemental areas of the second photoconducting layer by a path which conducts electrons, means for illuminating said first layer with radiations from an object, said second layer not being directly affected thereby, means on the side of said target remote from said illuminating means for scanning said second layer with a beam of light radiations, said first layer not being directly affected thereby, whereby the radiations from the object act on the rst layer to condition the arrangement and the scanning beam of radiations acts on the second layer, the two groups of radiations cooperating in the formation of an image current, a light transmitting metallic element in Contact with the surface of each of said photoconducting layers remote from the other photoconducting layer, and circuit connections between said metallic elements through which connections the image current passes, said circuit connections including a source of direct potential.

FRANK GRAY. 

